Variable lead compressor



Jan. 28, 1969 .1. w. GARDNER Sheet INVENTOR Q/QW/v WfiEP/YEZ %bflATroRNEYs QQ .gx K

Filed Oct. 28, 1966 J. W. GARDNER VARIABLE LEAD COMPRESSOR Jan. 28, 1969Sheet Filed 0M. 28. 19.66

INVENTOR fa/WV 5/?2 4 0/1/56 ATTORNEYS Jan. 28, 1969 J. w. GARDNERVARIABLE LEAD COMPRESSOR Sheet Filed 00 28, 1966 INVENTOR .j/v/v Wflan/5? ATTORNEYS Jan. 28, 1969 Filed Oct, 28. 1966 J. W. GARDNERVARIABLE LEAD COMPRESSOR Sheet 5 of 5 4500 Amaze-481 ATTORNEYS UnitedStates Patent M 3,424,373 VARIABLE LEAD COMPRESSOR Int. Cl. F04c 17/04,17/12 ABSTRACT OF THE DISCLOSURE A fluid compressor having a pair ofintermeshing rotors is provided with a continuously variable lead forthe lobes and gates of the inter-meshing rotors. The variable leadextends from an inlet end of the compressor to an outlet end of thecompressor, and the lobes of the male rotor, of the pair of rotors inthe compressor, have Wrap angles of less than 360 degrees. Theintermeshing relationship between the pair of rotors is such that asealing line is formed with a decreasing length from the inlet end ofthe compressor to the outlet end of the compressor. A method ofassembling a variable lead compressor is described.

This invention relates to an improvement in helical screw typecompressors, and in particular, it relates to a compressor of the typein which the helix has a continuously variable pitch or lead whichresults in improved compression characteristics.

Rotary type screw compressors are well known in the art, and commonlyinclude two or more intermeshing rotors positioned within a housing andprovided with some driving means for rotating the intermeshed rotorsrelative to one another. Such a structure is shown in Lysholm et a1.Patent 2,111,568, Mar. 22, 1938. Compressors of this type are used forcompressing gases and an inlet is provided for introducing gas into oneend of the housing for compression and movement axially to an outlet atan opposite end of the housing. Such rotary screw compressors may beoperated dry where no liquid is injected into the compression chamber,or wet where a liquid is injected for use in the compression chamber.Dry compressors rely upon close tolerances between intermeshing lobesand gates of the rotors to effect a seal between the rotors, and such asealing arrangement is commonly referred to as space sealing. The use ofliquids in a compression chamber of screw compressors provides foradditional sealing between the rotor clearances, and further providesfor removal of the heat of compression. A compressor which is operateddry typically has had a maximum compression ratio of 4:1, whereas thewet operated compressor, such as described in Bailey Patent 3,073,514,Jan. 15, 1963, may have a compression ratio of up to 9:1. Such prior artscrew compressors are constructed with constant lead lobes and gates onintermeshing rotors, which construction has necessarily limited theefficiency and performance of the compressors.

Some of the difiiculties in obtaining acceptable efficiencies at highercompression ratios in prior constant lead machines have resided inthrottling losses which occur because of small discharge ports and inhigh leakage losses in the high compression zones because the leakageline lengths are fixed. The leakage factor of a helical screw compressormay be related to the length of sealing line which is formed betweenintermeshed lobes and gates of two rotary members. By adjusting thelength of the sealing line for a given pair of rotors in a compressor, acontrol of compressed fluid leakage can be accomplished. This inventionhas found that the angles of a sealing line provide for a criticalcontrol of the leakage in a compressor, and a construction having a con-3,424,373 Patented Jan. 28, 1969 tinuously variable lead or pitch forthe lobes and for the gates of a pair of rotors provides a desiredleakage control. As a result of this improved construction, it has beenfound that compression ratios above 9:1 in a Wet operating compressor,and above 4:1 in a dry operating compressor, can be efficiently attainedin a machine having similar length and diameter ratios to prior machineshaving constant pitch ratios.

The compressor of this invention employs but two rotors, thus obviatingthe known disadvantages attendant constructions which employ :more thantwo. Moreover, the wrap angle of a compressor embodying this inventionis less than 360, again obviating greater manufacturing costs attendantgreater Wrap angles.

In addition to the above advantages, the compressor of this inventionprovides greater capacities for a unit of given size by increasing thedischarge area for the unit. Prior devices that have been designed toattain high compression ratios have necessarily included limited areadischarge ports which result in throttling losses within the compressor.In the present device of this invention, a higher compression ratio andcapacity can be attained with a given size of discharge port, and thisaccounts for substantially improved efficiencies.

A method of assembly will also be described with reference to theinvention, and the method includes the formation of thin, plated metaldiscs which are formed to provide the cross sectional configurations ofthe compressor rotors. A plurality of the discs are stacked together andarranged with a template to set the desired variable lead in the rotorcomponents. After the desired pitch is attained, with each disc beingslightly offset relative to the next one, the entire assembly can befused together by heating to a temperature which will melt the platingson adjoining surfaces of the discs. Finally the entire assembly may befinished and coated with a suitable material.

These and other advantages of the present invention will become apparentin the more detailed discussion which follows, and in that discussion,reference will be made to the accompanying drawings in which:

FIGURE 1 illustrates a perspective view of a pair of compressor rotorshaving variable lead intermeshing elements;

FIGURE 2 is a graph showing the improved displacement characteristics ofthe compressor of this invention as compared to a constant lead rotorconstruction;

FIGURE 3 is a graph showing the improved compression characteristics ofthe present invention as compared to a constant lead compressor;

FIGURE 4 is a cylinder and port development for a rotor pair;

FIGURE 5 is a complete cylinder and port development for a variable leadcompressor;

FIGURE 6 is a depiction of sealing line lengths as lead angle changesfrom one end of a rotor to another;

FIGURE 7 is a graphic description of sealing line length as compared tothe lead angle of a female rotor; and

FIGURE 8 illustrates a vertical cross section of a portion of a rotorwhich has been assembled by a plurality of stacked discs.

Compressors having interengaging screw rotors are well known in the art,of compressing gases, and the theory for forming a pocket of gas whichis compressed and carried from an inlet end of a unit to an outlet endof the unit is also well known. In the usual compressor design, theinterengaging rotors have wrap angles of less than 360 degrees andlength over diameter ratios (L/ D) ranging from 1.0 to 2.0. Suchconventional compressor units include rotors having a constant lead orpitch to their respective lobes and gates. It is also known in the priorart to provide for two stage compressors having two separate compressorelements combined together to form a single rotor wherein each of thecompressor elements has a different constant lead to its lobes or gates.Such constructions require an assembly of four separate elements toprovide for a two rotor compressor having two separate stages ofcompressing capacity.

The present invention provides for compressor rotor structures whereinthe rotors have continuously variable leads which result in an improvedcompression efficiency for the unit.

FIGURE 1 illustrates a perspective view of a set of rotor unitsembodying this invention for use in a compressor having a continuoussingle stage for compression. The rotor units in a given set comprise amale rotor and a female rotor 12, and the two rotors are mounted onparallel shafts 14 so that they may be driven in counterrotationaldirections to each other. When the rotors are driven, the lobes 16. ofthe male rotor 10 fit into the gates or grooves 18 formed within thefemale rotor 12, and the intermeshing of the two rotors causes acompression of a gas pocket trapped between intermeshing lobes andgates. The illustrated rotors are of the type wherein the male rotor 10has four lobes and wherein the female rotor 12 includes six gates 18.Further, the rotors are of the well known geometry providing for thetips of the lobes 16 to lie outside of the pitch circle of the malerotor and for the bases of the gates 18 to lie within the pitch circleof the female rotor. The lobe and gate configurations can be circular orgenerated, or any of the well known combinations or variationstherefrom. The compressor unit includes a conventional housing (notshown) that surrounds the two rotors so as to confine gas which isadmitted into an inlet end of the compressor unit. The housing structureand means for confining gases, such as the use of end plates inconjunction with the housing, are well known in the art and do not forma separate part of this invention. The set of rotors illustrated inFIGURE 1 may include gears 20 which engage with each other to providefor a timed rotation of one rotor relative to the other. The gears 20are normally incorporated in compressors which are operated dry,however, it has been found that gears may be entirely omitted from wetoperating compressors.

The lobes and gates of the rotors are formed in continuous variableleads which provide for a wrap angle of less than 360 for a lobe of amale rotor. Of course, the wrap angle of the female rotor corresponds tothe wrap angle of the male together with a consideration of the numberof lobes and gates used in a particular rotor set. When mounted in aconventional casing for use as a compressor, an inlet for the set ofrotors would be at the left end of the FIGURE 1 drawing, and an outletwould be at the right end of the drawing. Accordingly, gas would enterthe set of rotors at the end having the lesser lead for the lobes andgates, and the gas would be exhausted in a compressed state at the endof the set where the lobes and gates intermesh at a greater lead.

Substantial benefits are obtained from a compressor having rotors of avariable lead, as shown in FIGURE 1. The compressor of this inventionprovides for improved volumetric efficiencies in gas compression as aresult of (a) increased compression of a gas pocket due to the everincreasing lead for the lobes and gates as the gas pocket moves from aninlet to an outlet of the compressor, (b) less throttling losses as aresult of a larger discharge area which is available, and (c) decreasedleakage losses due to an improved sealing line characteristic. Theinvention provides for a compressor having an increased capacity for agiven length over diameter ratio (L/D) of a unit, and also increasedcompressions are efliciently attainable within such a compressor unit.The above advantages and benefits will be discussed with reference t9the graphic illustrations of FIGURES 2 through 7 FIGURES 2 and 3 comparedisplacements and cell pressures for compressors having constant leadrotors and variable lead rotors of a type made in accordance with thisinvention. The FIGURES 2 and 3 analyze only one pressure cell in acompressor having a four lobe male rotor and a six gate female rotor.There are four separate compression cycles per revolution of the maledrive rotor, but of course it is understood that the number ofcompression cycles per revolution would vary with the rotor ratio (malerotor lobes to female rotor gates) and/ or the particular rotor which isutilized for driving. Referring to FIGURE 2 there is shown adisplacement-time diagram of one cell of a conventional constant leadscrew rotor set as compared to a variable lead rotor set of the typeconstructed in accordance with this invention. The rotor sets selectedfor illustrating the displacement-time diagram of the variable andconstant lead compressors are dimensionally similar in that the rotorlengths, rotor diameters, rotor wrap angles, theoretical displacementand rotor profile configurations are substantially identical. With theFIGURE 2 graph the port locations for the compared compressors can bedetermined in a well known manner (as is known for compressors of thefixed port type wherein there is a built-in internal volume ratio whichmay be defined as the ratio of suction port cut off volume to dischargeport expelled volume). For example, FIGURE 2 shows the total cell (maleand female) theoretical displacement volume for constant lead andvariable lead rotor sets to be 172 cubic inches. Assuming that gas beingcompressed is air (n: 1.4) and that the compression cycle follows verynearly to the adiabatic compression cycle, the volume required at thedischarge port to obtain a discharge pressure of 114.7 p.s.i.a. orp.s.i.g, would be 39.6 cubic inches. This relates to a volume ratio of4.3 to 1. By a well known thermodynamic formula this can be illustrated:

Applying this formula to the theoretical displacement diagram of FIGURE2, pressure time cards for the constant lead and variable lead rotorsets can be constructed and compared as shown in FIGURE 3.

By analyzing FIGURES 2 and 3, it is readily apparent that the variablelead rotor set cell will reach the required volume ratio and its relateddischarge pressure of 114.7 p.s.i.a. approximately 52 degrees earlierthan the standard lead rotor set cell. Since the total wrap angles ofboth rotor sets (constant lead and variable lead) are identical, thetime remaining to expel] the gas is greater. This time difference isdirectly related to the discharge port size. FIG- URES 2 and 3 have beenmarked to show discharge port locations for 114.7 p.s.i.a. for the twotypes of rotor constructions when operating on air (n=l.4) and followingan adiabatic compression cycle. It is clear from the FIG- URES 2 and 3that a greater discharge port area for higher discharge pressures areavailable in the variable lead rotor set built in accordance with thepresent invention; and as will be discussed below, throttling losses arereduced for a compression unit utilizing this arrangement.

In order to further illustrate the benefit of increased discharge portarea which is obtained with the present invention, FIGURE 4 illustratesa typical diagramatical layout of a cylinder which surrounds a rotor setand a discharge end plate for a compressor. The FIGURE 4 diagram is acylinder and end plate port development for constant and variable leadrotor sets having axial and radial discharge port locations. Of course,it is understood that the rotor sets may be in compressor units havingonly axial ports or only radial ports, as are well known in the art. TheFIGURE 4 development includes an identification of the suction anddischarge ports which are determined from the information of FIGURES 2and 3. Again, the illustrated rotor pairs are of the type wherein the mle lOI' includes four lobes and the female rotor includes six gates orgrooves, however, similar developments could be made for other wellknown rotor configurations and rotor ratios. In the FIGURE 4 comparison,the exhaust port areas for a conventional constant lead rotor set isidentified by the strippled-sections shown on the end plate developmentand on the cylinder development immediately below the end plateillustration. The comparable discharge port area for a variable leadrotor set is identified by all of the stippled area together with thecross hatched section which is shown. It is readily apparent that thevariable lead rotor set will provide a substantially greater dischargeport area, and this results in lower throttling losses in a compressorunit. Throttling loss, as is well known in the art, considers the amountof pressure backup within a compressor unit which results from theinability of compressed gas to be easily and quickly discharged. As alarger discharge port area is made available, the throttling or backuppressures will be diminished and there will be less throttling leakagewithin the compressor unit.

FIGURE 5 shows a complete cylinder and port development of the typeshown in FIGURE 4, but illustrating a variable lead rotor set only. Thedevelopment of FIGURE 5 also shows the locational relationships of themale and female rotor outside diameters with respect to the surroundinghousing.

FIGURE 6 illustrates a further advantage obtained from the variable leadconstruction of this invention. In prior rotor type compressors of theconstant lead type, the compression efficiencies have been depended uponthe geometry of the set of intermeshed rotors used in a given compressorunit. The geometry includes such variables as the number of lobes andgates, length and diameter of the individual rotors, and the wrap anglewhich is utilized with respect to the lobes and gates formed onindividual rotors. Assuming much of the geometry of the present deviceto be comparable to prior art types of rotor sets, a further variable isadjusted in the present invention to improve the efficiency of a rotorpair in a compression operation. This variable is concerned with thecompression cell sealing line length, and the length of sealing line isdirectly related to the amount of leakage loss which may occur within acompressor unit. The sealing line of a rotor set can be considered theline of closest proximity between intermeshed lobes and gates in theset. Since the rotors are not in actual contact with each other at anytime, the sealing line represents the closest point of contact and isdeterminative of the amount of leakage which will occur betweenintermeshed rotors in a compressor construction. The heavy line A-G ofFIGURE 6 represents a sealing line configuration at the intake end of arotorpair made in accordance with this invention; and the dashed lie agrepresets a sealing line configuration at an exhaust end of a rotor pairmade in accordance with the present invention. It can be seen bycomparing the sealing line configuration at the intake end of the rotorpair to the sealing line at the exhaust end that there is a substantialreduction in sealing line lengths as the lead angle of a rotor pairvaries. Thus, in accordance with the present invention there is providedan ever decreasing sealing line length between intermeshed rotors fromone end of the compressor unit to the other. The reduction in sealingline length takes places toward the critical end (that is, the exhaustend) of the unit where greater pressures are developed within thecompressor cells, and therefore, gas leakage between intermeshed rotorsat this critical portion of a compressor unit is substantially reduced.The length of sealing line is a critical factor in the performance andefficiency of a unit, regardless of whether the compressor is operatedwet or dry, and the reduction in sealing line length toward an exhaustend of a unit results in an improved efliciency of the compressor unit.The FIGURE 6 comparison is taken at the extreme ends of a rotor setconstructed in accordance with the present invention, but it is to beunderstood that the sealing line length is a continuously changingfactor because of the continuously variable change in lead angle of therotor pair built by the present invention.

Sealing line lengths in a conventional constant lead compressor remainconstant throughout the length of a given compressor unit, and ofcourse, this means that there is no reduction in leakage betweenintermeshed rotors as the gas pocket is compressed from the intake endto the exhaust end of a unit. Such is not the case with the presentinvention, since the sealing line length reduces as the gas pocket iscompressed toward the exhaust end of the unit. FIGURE 7 furtherillustrates the sealing line length relationship to varying lead anglesof a typical female rotor construction as the lead angle changes fromone end of the rotor to the other. It is apparent from the FIG- URE 7graph that there is a considerable and continuous reduction in thesealing line length from the intake end of the rotor to its exhaust end.Of course, it is understood that the illustrations of FIGURES 6 and 7are by way of example only, and that numerous rotor profiles exist whichhave different numbers of sealing lines and different configurations forsealing lines. Also, it is possible to plot the sealing line as afunction of the male rotor lead angle instead of the illustrated femalerotor lead angle.

Having described the constructional features of the present invention,and having depicted the benefits obtained thereby, it can be seen thatthe provision of a continuously variable lead for a rotor pair providessubstantially improved compression results. The continuously variablelead for intermeshing rotors provides for a more rapid compression of agas pocket, resulting in a capability of discharging the compressed gasin a higher state of compression. Also, the improved construction ofthis invention provides for a larger discharge port area for a givencompressor unit, and this results in less throttling losses within theunit from a backup of compressed gas between intermeshed rotors.Finally, the construction of this invention provides for an improvedsealing line characteristic which becomes shorter as a gas pocket ismoved from an inlet end to a discharge end of a compression unit. Theshortening of the sealing line results in less gas leakage betweenintermeshed rotor pairs.

As an example of an operating construction for a compressor unit of thisinvention, rotor sets can be made which include length over diameterratios within the usual range of 1.0 to 2.0. The male rotor can berotated at a lobe tip speed within a range of 60 to 125 feet per second,and the female rotor is operated at a speed to accommodate the malerotor speed. In such a unit, the compression reached approximately 230p.s.i.a. as compared to approximately p.s.i.a. on a conventional unit ofsimilar dimension but having constant lead lobes and gates. Thisdifference in compression capacity can be attributed to a greaterprogressive compression of a fluid pocket and also to improvedefiiciency resulting from lower leakage losses in the unit of thisinvention.

The rotor set of this invention may be formed by any suitable methodsuch as by casting or cutting the required lobes and gates into theseparate rotors, or by machining the same. By way of example theindividual rotors may be formed from an assembly of stacked plates whichare mounted face to face along the central longitudinal axis of therotor to be formed. Each succeeding plate in the assembly is slightlyoffset relative to other plates, and the amount of offset determines thechange in lobe or gate pitch which is required for the construction.FIG- URE 8 illustrates a vertical section taken on line 8-8 of FIGURE 1,and this section illustrates a series of adjoining plates 30 which areassembled side by side to form the desired structure for a rotor. Thestacked plates may be initially mounted on an arbor and cut to provide adesired profile and lead angle, as would be formed in a standardcompressor rotor having a constant lead rotor construction. Existingcutting devices can be used for forming such a constant lead and profilein a rotor surface. Then the discs can be removed from the arbor andremounted on cores 32 where they are displaced into a continuouslyvariable lead of the required pitch. A template may be used fordisplacing or setting the stacked discs into the desired variable helix.Then the entire assembly can be heated to the melting temperature of thesurface of the individual discs, and the entire assembly will be fusedtogether into a solid structure. In order to facilitate brazing ormelting of adjoining surfaces of individual discs, it has been foundthat an improved unit can be formed by coating the assembly, prior toheating, with a silver base alloy. The silver base alloy may be appliedin a fluid form so that capillary action will carry the alloy betweenadjoining discs of the assembly. This provides a coating on theadjoining surface of the disc, and the heating stepthen results in asolid bonding of adjacent disc to one another. Finally, the assembledrotor may be coated with molten tin, or other material, to rovide asmooth surface On the rotor configuration which is formed. It has beenfound that such coating materials as epoxy resins and metal alloys canbe added in a fluid state while the rotor pair is being slowly rotatedtogether and intermeshed. This method has resulted in an improvedfitting of the lobes and gates of the rotor pair to one another in thefinished set.

Alternatively, the variable lead can be formed from stacked discs whichare not staggered in a progressive fashion relative to one another butwherein each disc has a slightly different helix angle in its profile.The assembly of such discs would result in a predetermined variable leadangle, and the bonding techniques could be of the type described above.

Although the term gas has been used throughout the above description,such language contemplates a change of state of certain gases which maytake place during compression, and therefore the term may include liquidcondensation or liquids generally. Also, other changes and obviousvariations may be made in the invention and such variations are intendedto be included within the scope of this invention.

What is claimed is:

1. A fluid compressor having improved compression characteristics and animproved availability of an increased size for an outlet port leadingout of the compressor, said fiuid compressor being of the screw typehaving only a pair of complementary intermeshing cylindrical rotorsmounted in a housing having cylindrical chambers for receiving said pairof rotors and for controlling the transfer and compression of fluidsfrom an inlet into one end of said housing to an outlet from an oppositeend of said housing, the improvement in said rotor pair which comprises:

a male rotor having a plurality of equally spaced lobes projecting fromits cylindrical surface, said lobes extending from one end of the rotorcylinder to the other end of the cylinder along equally spaced pathswhich continuously vary in lead as they progress along and around thesurface of the rotor, said lobes having wrap angles of less than 360degrees and a female rotor having a plurality of equally spaced gates orgrooves formed in its cylindrical surface, each of said gates beingformed to complement and receive a lobe from said male rotor when saidrotor pair is mounted in intermeshing relationship in a compressor unit,said intermeshing relationship being such that a sealing line formedbetween the intermeshed rotor pair decreases in length from the inletend of the compressor to the outlet end thereof.

2. The fluid compressor of claim 1 wherein the variable lead of thelobes and gates for each of said rotors progresses from a relativelyslight angular relationship to a central longitudinal axis of anassociated rotor to a relatively steep angular relationship to saidassociated central longitudinal axis as the lead varies from an inletend of said rotor pair to an outlet end of said rotor pair, wherebypockets of fluids are progressively compressed as they are transferredfrom the inlet end to the outlet end upon rotation of the intermeshedpair of rotors.

3. The fluid compressor of claim 1 wherein the male rotor is rotated ata lobe tip speed within the range of approximately feet per second toapproximately feet per second.

4. The fluid compressor of claim 1 wherein the lobes of the male rotorlie outside of the pitch circle of said rotor and wherein the gates orgrooves of said female rotor lie within tthe pitch circle of said femalerotor, said lobes and gates intermeshing to form compression chamberswhich compress and confine fluid pockets in a progressive movement ofthe fluid from an inlet end of the housing to an outlet end of thehousing, and said cylinder chambers within said housing furtherconfining said pockets of fluid to compression chambers formed by therotation of said rotor pair.

5. The compressor of claim 4 wherein each lobe of said male rotor has aconvex surface configuration which includes two apexes which generatetwo spaced thread points relative to the concave base of a complementarygate.

6. The compressor of claim 1 wherein said rotors are constructed with aratio of length to diameter (L/D) within a range of 1.0 to 2.0.

References Cited UNITED STATES PATENTS 630,648 8/1899 Brewer 91842,148,205 2/1939 Kiesskalt 103l28 2,804,260 8/1957 Nilsson et a1. 2301433,314,597 4/1967 Schibbye 230143 DONLEY J. STOCKING, Primary Examiner.

WILBUR J. GOODLIN, Assistant Examiner.

@3 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,2 ,373 Dated January 28, 1969 Inventofls) JOhI'l W. Gardner It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 3, line 57, "lesser" should be corrected to read --greater--;

line 59, 'greacer" should be corrected to read --lesser--.

SIGNED AND SEALED a new mm-nu E. 50mm, m. Awning Officer Gemisaionor ofPatents

